Issue with using segments in Google Analytics
I have set up a series of segments to help filter visitors to the blog section of a client site - for example I have a segment that includes all visits that include a blog page view. The problem is that the data i see for 'all sessions' changes when I introduce an additional segment - why would this be?
For example if i am just looking at the 'all sessions' segment - it might tell me that the page /blog/pageone.html has had 150 page views. When I introduce an additional segment (eg. for visits that include a blog page view) and do the same search for /blog/pageone.html, both segments (including 'all sessions') now show different values. Why would the 'all sessions' segment show different sets of results just because it sits alongs side another segment?!
Why would this be?! Really confused!
Thanks in advance for your help!
Re: Issue with using segments in Google Analytics[ Edited ]
November 2015 - last edited November 2015
that should be a sampling issue very probably. If you don't apply any segment having 100% sessions the standard reports in Analytics (e.g. the basic "All pages" report) are always unsampled.
If you apply a segment, however, an actual live drill-down in the data is being performed. In that case, if you're considering more than about 500,000 sessions at a glance, sampling of data occurs  - you see that yellow bar on the top right. Then both the 100% segment and your proper segment are sampled which in turn changes the exact values you see in an unsampled report into mere estimates. That's the change in numbers you've noted.
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